Digital cameras sample the light that bounces off a subject to create a digital image. A digital camera may use a shutter in combination with a sensor to determine an exposure time and acquire the proper amount of light to make a good image. The proper amount of light may come from accumulating ambient light over time until the proper amount of light is obtained. Alternatively, the addition of artificial light from a flash reduces the amount of time to obtain the proper amount of light to make a good image.
A pre-flash may be used to predict the amount of flash needed during the exposure of the picture to obtain good image quality. The illumination using pre-flash and ambient light can be compared to illumination with ambient light alone to determine the amount of flash illumination needed during exposure of the picture.
Some digital cameras use a different image sensor for taking the picture with the flash and for measuring illumination during the pre-flash time period. A sensor array such as a complementary metal-oxide semiconductor (CMOS) sensor or a Charge Coupled Device (CCD) sensor may have a limited dynamic operating range in order to make a good image.
CMOS sensors implementing a rolling shutter timing mechanism may have a relatively slow exposure time when a flash is required. Often, in a scene having ambient light, the slow exposure time results in a blurry photograph for a hand-held camera due to camera motion during the time the shutter is open. In a sensor implementing a rolling shutter, the light accumulating pixels in the sensor both track the accumulation of charge and are read out in a pixel line by pixel line manner. Since the integration process moves through the sensor pixel lines over some length of time, some motion artifacts may become apparent. For example, if a vehicle is moving through the image during capture, then light from the top of vehicle will be integrated at some earlier time than light from the bottom of the vehicle, causing the bottom of the vehicle to appear slanted forward in the direction of motion.
Some digital cameras have a mechanical shutter mechanism used during the flash to reduce the exposure time of the CMOS sensor and reduce blurring of the image. Unfortunately, the typically long exposure time during the pre-flash can lead to an inaccurate prediction of the flash needed for taking the picture with a short exposure time. The CMOS sensor with a limited dynamic operating range may become clipped or flooded with light and produce poor images in response to a non-ideal flash.